A bicycle, especially a sport bicycle, such as a mountain bicycle, a road bicycle, or the like, usually has a complex speed changing system, including two cogsets, i.e., a front cogset attached to the crankset of the bicycle and a rear cogset attached to the rear wheel. Each of the cogsets can include a plurality of sprockets with different sizes. Sprockets in the front cogset, also referred to as “driving sprockets,” and sprockets in the rear cogset, also referred to as “driven sprockets,” can be freely combined to meet different road conditions. The combinations of the driving sprockets and the driven sprockets can be controlled by a gear shifting set including two shifters each controlling one of the cogsets. A user needs to manually switch the shifters to achieve speed changing. This increases the complexity in operation and control, making it difficult for non-professional cyclists, i.e., ordinary cyclists to perform gear shifting operations.